About 19,000 pelicans returned this spring, and it appeared things were back to normal for the largest white pelican colony in North America. The pelicans nested and perhaps up to 9,000 chicks hatched.

Then last week wildlife officials discovered most of the chicks dead, and most of the adults gone -- again. About 500 chicks and 2,000 adult pelicans remain.

"It's not quite lightning striking twice in the same place, but it's maybe lightning and then a tornado striking," said Ken Torkelson, spokesman for the U.S. Fish and Wildlife Service. "We're scratching our heads plenty right now."

Added Torkelson: "We had hoped we were on the road to recovery or normalcy at Chase Lake; this is certainly a setback."

Officials don't believe the two incidents are related. But they have no definite answers to explain the dual mysteries.

There are key differences between the two events.

"Last year, the adult pelicans took off, leaving their nests and eggs behind," Torkelson said. Few eggs had hatched.

"This year, virtually all of the eggs hatched, the young died and then the adults took off."

Disease is the likely culprit for the dead young pelicans this year. Samples have been sent to the National Wildlife Health Center in Madison, Wis., and results may be available this week or next.

One theory: The young birds' immune systems might have been stressed by wind and rain storms that have peppered the area, along with high temperatures.

"They were big enough so they were no longer in the nests for the parents to shelter and protect them," Torkelson said. "We don't think the weather killed them, but it maybe stressed their immune system; once that is down, all kinds of bad things can happen."

Scientists still don't know what caused last year's abandonment. Disturbance -- by humans or predators -- remains a possibility. This year, human access was restricted more than usual and fences were erected to keep out predators such as foxes and coyotes.

"It's still entirely possible that last year's abandonment was a quirk of nature, one of those strange occurrences that never gets explained," Torkelson said.

But officials know where some of this year's adults went.

Seven or eight birds were affixed with satellite transmitters, so officials could track their movement. "Most of them are in and around North Dakota. Some are close to Chase Lake, but they have no reason to go back because they don't have young there anymore," Torkelson said. He said no additional chick deaths were observed Monday or Tuesday, "so that's really a good sign."

Though this will be the second consecutive year of poor pelican reproduction, the white pelican population isn't threatened by the two incidents, Torkelson said.

"Are we worried about the demise of the continental population of the white pelican population? Probably not.